Constant temperature device



3, 1966 R. E. MANNING ETAL 3,267,687

CONSTANT TEMPERATURE DEVICE Filed Sept. 22, 1964 2 Sheets-$heet 1 FIG.

INVENTORS ROBERT E. MANNING WALLIS A.LLOYD Aug. 23, 1966 R. E. MANNINGETAL CONSTANT TEMPERATURE DEVICE 2 Sheets-Sheet 2 Filed Sept. 22, 1964Mac-99 9 INVENTORS ROBERT E. MANNING WALLIS A.LLOYD United States ate t3,267,687 CONSTANT TEMPERATURE DEVICE Robert E. Manning and Wallis A.Lloyd, Boalshurg, Pa., assignors to Cannon Instrument Company,Boalsburg, Pin, a corporation of Pennsylvania Filed Sept. 22, 1964, Ser.No. 398,349 5 Claims. (Ci. 62-168) This invention relates to a constant,low temperature bath device for conducting experiments, such asviscosity measurements, at a low temperature which is closely controlledwithin narrow limits.

A related bath or bath device is described in copending application Ser.No. 261,174, filed February 26, 1963, now Patent 3,214,937, issuedNovember 2, 1965. Like that device, the present bath is of value fortesting lubricants, fuels, hydraulic fluids, and the like at lowtemperatures. For example, military installations in arctic climateshave generated much interest in the viscosity behavior of lubricants andother fluids at reduced temperatures, and as a natural consequence,considerable attention has been shown to instruments and apparatus formeasur ing viscosity at these temperatures. Conventional refrigerationequipment can, of course, be used to hold a test material at lowtemperature, but it tends to be quite bulky and to require valuablespace; and furthermore, such equipment is expensive, complicated toinstall and operate, and apt to require expert maintenance.

According to the invention, there is provided a cooling device which isof desirable simplicity and sound operation, like the bath of said priorapplication; additionally it alfords unique advantages which enhance itsutility. More particularly, it is characterized by an improved coolingsystem which enables the cooling rate to be closely controlled withoutreliance on an added heater; this means that only necessary heat ismmoved, avoids overcooling, and aflords economies in the use of coolant.Furthermore, space is conserved, and the compactness of the device isincreased. compactness is also favored by the disposition of the coolantreservoir and the bath container, these being arranged to have theirfluid levels at more or less the same heights, so that accessibility aswell as compactness is improved. Other advantages will become apparentfrom the ensuing description.

The present device comprises generally a bath section for holding bathfluid to be cooled and maintained at a constant low temperature, areservoir section for holding fluid coolant, and a central sectionintermediate the other two. Temperature control means, suitablycomprising electrical switching circuits, are associated with thecontrol section. The bath and reservoir sections, as also the centralsection, have the same floor level, and as noted, the first two sectionshave substantially the same fluid levels. Temperature-sensitive meansextend into the bath fluid and serve to actuate said control means.

Fluid coolant passes between the reservoir and bath sections, suitableconduit or transfer means being provided for passing coolant from onesection to the other, preferably in a lateral direction, and back again,also preferably laterally. In the bath section most of the coolant flowis preferably in an up-and-dorwn direction, although it may be in otherdirections also, including a spiral, zigmg, or other circuitous route.Valve means are provided in the coolant transfer means which areresponsive to the temperature control means for controlling flow ofcoolant.

When the temperature-sensitive means in the bath fluid signals a highbath temperature, the electrical switching circuits or means areactuated to operate the valve means, thereby to open the latter topermit coolant flow theret-h-rough; and when the bath is sufficientlycooled, the temperature-responsive means signal to inactuate theswitching circuits and the valve means close to shut off coolant flow.Overcooling of the bath does not take place, as will be described.

While a number of coolants are suitable, commercial Dry Ice, or frozencarbon dioxide, is admirable, and the invention may be described indetail in connection with this material. The Dry Ice is placed in chunksin the reservoir together with a small amount of a solvent therefor.Cooled solution, comprising solvent together with some dissolved DryIce, flow from the reservoir into the transfer means, which extendsthrough the bath, Where the solution cools the bath fluid by indirectheat exchange. Heat abstracted from the bath boils out dissolved carbondioxide from the solution, and the gas and some liquid flow on throughthe conduit means and back to the Dry Ice reservoir. In turn, additionalcold solution flows from the reservoir to the bath via the conduit meansbetween them and in this way the temperature of the bath may be broughtdown to a desired low temperature, at lWh'lC'h point the flow of gas andliquid through the conduit means is reduced, this in turn reduces theflow of cold solution from the reservoir into the conduit means, and aline control temperature-regulating system is actuated to maintain thetemperature.

The invention may be better understood by referring to the accompanyingdrawings, which are diagrammatic, and in which:

FIG. 1 is a perspective view of the constant temperature device, withportions broken away; and

FIG. 2 is a circuit diagram of a switching system.

Referring to FIG. 1, the constant temperature device comprises a bathcontainer 10 supported in a cabinet '11 which encloses the container onall sides. The container, which is in the form of an open top vessel ofglass or stainless steel or other suitable material, is disposed on oneside of the cabinet and is covered by a lid 12 which provides a workingarea for placing test equipment in the container. The latter is nearlyfilled with bath liquid 13 comprising the liquid which is to be cooledand maintained at constant temperature. On the other side of cabinet 11is a reservoir 14 comprising a thick-walled tank for holding coolant.The tank is insulated on all sides by walls of insulating material, asat 15 and 16, as is container 10 by walls 9 and 17. The floor '18 andlid '19 are similarly protected by or constructed of insulatingmaterial.

The coolant in reservoir 14 comp-rises chunks or blocks 29 of Dry Iceand are immersed in a Dry Ice solvent 21 such as methanol or acetone.The blocks rest on .a screen partly indicated in section at 22 which isdisposed a small distance above floor 18, say A to 1 inch, preferably /2to inch, and thus solvent has free access to the blocks. However, thescreen is dispensable. The level of solvent in the tank may besutlicient to cover the blocks, as shown, or may only partially coverthem.

In the control part of cabinet 11 there is disposed a compartment 25between the two side compartments for the disposition or support ofcontrol means comprising a conduit member or coolant flow tube 23, anelectric control valve 30 (shown partly in section), a hand throttlevalve 31, and, disposed above the compartment, although it maybe locatedin any suitable position, near or remote, an electrical switching system43. The coolant flow tube 23 extends horizontally into the reservoir 14a short distance where its inlet end, which is disposed well below thelevel of coolant 21, is provided with a screen 24; and opposite such endthe tube extends horizontally into the container 10, ivlhere it turnsand extends vertically, as at 26, then turns again to a horizontaldisposition and reenters compartment 25, note section 27, and finallyextends again into reservoir /14 by means of outlet 28 disposed abovethe level of coolant 21. Outlet 28 may also tbel-oany case it is lZlbOVthe inlet 24.

Lid 12 is provided with several openings, such as at 40, for receivingconventional instruments like viscometers. Another opening is occupiedby a motor-operated stirrer 41, and another by a conventionaltherrnoregulator 4-2 shown with leads 58 and 59 connected thereto whichextend into the switching system 43. Another pair of leads 80 and 81 areshown connecting the coil of the solenoid valve to the system 43.

A side panel is adapted to cover the side of the device nearest theobserver, and in the bath compartment 10 this panel suitably has awindow 36 to enable the op erator to watch the progress of tests. It ishelpful to provide a light to aid visibility and a small electric heaterto prevent frosting and condensation on the window; these are not shownfor sake of a simplified showing. Also not shown are a pump andconnections for removing bath fluid from container 10 and circulatingthe same to equipment external to the container.

The operation of the device may be described briefiy as follows.

Coolant .21 in reservoir 14, comprising, for example, a methanol-carbondioxide solution, will flow into tube 23 through inlet screen 24, thevalves 30 and 31 being open for this purpose, and will reach a levelindicated at 32 in section 26 of the tube, this level corresponding tothe level in reservoir 14. The bath liquid 13, for example methanol,transfers heat to the cold solution in tube 26, causing carbon dioxideto boil out of solution, and this gas will push or entrain some methanolthrough section 27 of the tube in the direction of arrow 94, the mixturepassing forcefully through the open valves 30 and 31 and into reservoir14. It may exit from tube 27 with such force as to strike the oppositewall of the reservoir. Because of the pressure differential created,additional saturated solution from the reservoir enters inlet 2- of thetube. Thus, if valves 30 and 31 remain open, circulation is continuousand the bath liquid 13 will cool.

When the bath cools to a desired control temperature, thethermoregulator 42 operates through switch system 43 to close thesolenoid valve 30, this action being described. below. Carbon dioxidecontinues to boil out of the liquid in tube 26 but now the pressure inthis tube above the liquid builds up rapidly, forcing the liquid backthrough inlet 24 into the reservoir. Hence, the cooling ceases, and thebath begins to warm from heat transfer from the surroundings. By virtueof the rapid action of the cooling cessation step, undercooling(sometimes designated overcooling) is avoided and fine control of thebath 7 temperature is possible. It will be noted that cessation ofcooling is self-regulated, i.e., a self-ceasing cooling action isinvolved.

When the temperature moves slightly above the control point, thethermoregulator again signals valve 30 to open,

the pressure in tube 26 is reduced, cold solution again. fills tube 26,and cooling commences again. In this manner the bath can be held towithin slight variations above and below a desired temperature.

Throttle valve 31 can be adjusted such that the rate of cooling morenearly approximates the rate of heat gain from the surroundings. Thistends to reduce the degree of cycling of the temperature. Themotor-stirrer 441 tends to establish a uniform temperature throughoutthe bath. Carbon dioxide gas is vented from reservoir 14 by means notshown.

From the foregoing operation, it may be apparent that fiow tube 23comprises the inlet portion, the tube 27 the outlet portion, and thetube 26 the heat transfer portion of the coolant transfer means. Portion26 thus com prises coolant flow transfer means as well as heat transfermeans, and is constructed of suitable heat conducting material.

The desired low temperature of the bath will depend in part on thesolvent, but with this in mind, a wide range of temperatures isavailable, extending for example from just below room temperature downto about l00 degrees Fahrenheit. The bath liquid 13, of course, is onethat is liquid and stable at the chosen temperature, and may includewax-freehydrocarbons, acetone, methanol, etc. If desired, the bathliquid may be the same as the carbon dioxide solvent, described below.

Accurate control over the bath test temperature is possible, thevariation being less than plus or minus 0.04 degree F. at bathtemperatures of O to -65 degrees F., and in many cases the variationbeing no more than plus or minus 0.01 degree F. For bath temperaturesbetween room and 0 degree F. the variation may be less than 0.02 degreeF.

The carbon dioxide solvent may suitably be chosen from low molecularweight compounds including alcohols, ketones, ethers, esters, aldehydes,organic acids, etc. Low molecular weight compounds are desirablebecause, in general, their viscosity is low and they flow easily at thelow temperatures involved. Illustrative specific solvents are methanol,ethanol, propanol, isopropanol, n-butanol, isohutanol, acetone, methylethyl ketone, methyl acetate, ethyl acetate, propyl acetate, butylacetate, methyl butyrate, propyl butyrate, acetaldehyde,propionaldehyde. Other compounds are the various pentanes of hexanes,and methylene chloride.

The lateral position of the Dry Ice tank 14 relative to the bath 10 is aconvenient one for replenishing Dry Ice and increases the compactness ofthe device. It will be understood that the tank :may have otherpositions relatively to the bath and that forced flow of the Dry Icesolution may be employed. Also, the tube or member 23 may be spaced atvarying heights above the floor of compartment 25, resulting in varyinglengths of the cooling section 26 in the bath. Tube 23 may have anysuitable crosssectional shape. It will be appreciated that the rate ofheat transfer increases with increasing surface area, all other thingsbeing the same. Any suitable material may be used to construct the tube23, but preferably the material is a metal like stainless steel, brass,aluminum, copper, nickel, Zinc, iron, titanium, molybdenum etc.

Turning to FIG. 2 the thermoregulator 42 is a conventional contactthermometer comprising two sections, an upper section 43 for setting thedevice to the desired temperature, and a lower section 44 for readingthe actual temperature. Each section has its own temperature scale,indicated at 45 and 46, both being the same. In upper section 43 thereis disposed an adjustable rotatable screw-threaded electrode 47 having along thin wire 43 attached to its lower end which extends into lowersection 44. A marker 49 is attached to electrode 47 and is visible inupper section 43; it indicates the temperature on the up er scale 45 atthe same time that the lower end of the wire 48 indicates the sametemperature on thelower scale 46. Electrode 47 is rotated by turningknob 50 at the top of the device. The lower end 51 of the devicecomprises a bulb of mercury (preferably mercury-thallium) and isdisposable in the bath liquid 13. When the liquid warms, the mercuryexpands and the mercury column 52 reaches the set temperature, therebymaking contact between the mercury and the end 53 of the wire id, andthis closes a circuit. Thus, the device 42 acts as a switch.

The electric circuits which are energized by the thermoreguiator, andwhich control the operation of the solenoid valve 30, may be describedbriefly.

Consider that the bath liquid 13 has become heated to a temperatureabove the desired level, and that as a re suit the mercury column 52expands, making contact at 53 with the lower end of the wire 48. Thiscloses a circuit in the thermoregulator, which then operates theswitching system generally designated as 43, note FIG. 1, to which theleads 58 and 59 are connected. In turn, a circuit is established,designated 58-59, note FIG. 2, involving lead 59, resistance 60 (47,000ohms), resistance 61 (1 megohm), lead 62, secondary coil 63 (6.3 volts)of isolation transformer 64, diode 65, and lead 58. At point 66 ofcapacitor 67 (5 mfd.), the potential is volts, or any suitable value inthe range of -4 to volts. This follows from the fact that secondary coil63 of transformer 64 supplies alternating current which is rectified bydiode 65 so that capacitor 67 is charged at point 66 by a negativepotential, suitably about -l0 volts relative to the point 660. The coil63 also supplies current to the heater 69 of thyratron 71 (2D21). It isthe function of circuit 58-59 to transfer the potential at 66 to thepoint 68 through the current limiting resistor 60. The latter limits thecurrent through thermoregulator 42 to values preferably below 1 ma.

Point 68 is at the control grid of the thyratron 71, and the latter willnot conduct when the control grid is at a potential of -10 volts, andtherefore no current will flow in coil 72 of solenoid 73 in the relay74, and consequently movable switch 75 will be in contact with point 76.A circuit, designated 75-91, and involving switch 75 and point 76 shouldnot be considered. It comprises switch 75, point 76, lead 77, contact Cof terminal strip 79, lead 80, the coil of solenoid valve 30, lead 81,contact 2 of terminal strip 79, lead 83, contact 2 of strip 79, powersource (115 volts, 60 cycles) when switch 86 is closed, lead 87, contact1 of the strip, leads 89, 90, and 991, and back to switch 75.Establishment of the foregoing circuit, by closing switch 86, energizesthe coil in solenoid valve 30, and the plunger 92 is thereby raised toopen the valve 93 and thus permit coolant to flow in pipe 26 in thedirection of arrow 94, note FIG. 1. The flow of coolant acts to cool thebath 13 so that its temperature decreases.

Cooling of the bath, in time, results in opening circuit 58-59 at thepoint 53 owing to contraction of the mercury, note FIG. 2. The negativevoltage of -10 volts is thus no longer imposed at point 68; in turn thecapacitor (0.1 mfd.) begins to discharge through the side 101 andresistor 61, and the voltage at point 68 is raised to a valve of about 2volts, or even higher, going to 0 volt. When point 68 is at -2 volts,thyratron 71 will conduct current when its plate 102 is positive withrespect to cathode 103. This current energizes coil 72 of solenoid 73,and as a result switch 75 is drawn away from point 76, thus openingcircuit 75-91 at the latter point. (Switch 75, during the foregoingaction, will make contact at point 104, but this is of no concern to theinvention.)

As a result of the opening of circuit 75-91, solenoid 30 is no longerenergized, and therefore plunger 92 is moved by action of spring 105 toclose the valve 93 so that coolant fiow through pipe 26 is stopped. Atthis point of the operation, bath 1*3 begins to warm up.

In connection with the plate 102 of the thyratron, this is periodicallymade positive by the alternating voltage supplied by secondary coil (117volts). In other words, the alternating voltage is positive on everyother half cycle but negative on each following half cycle. Thethyratron does not conduct when the plate is at a negative potential. Inorder to keep coil 72 of solenoid 73 energized during the time the plate102 is negative,

eliance is placed on the inductive effect of this coil, which acts tokeep current flowing through it, and through the diode 111, during thehalf cycle when the plate is negative.

The terminal strip 79 represents a convenient means of connectingtogether the thermoregulator, power supply, solenoid valve, and, to theright of the strip, the relay circuits. The thermoregulator is connectedin at the contacts T and T; the power supply is connected at contacts 1and 2; and the solenoid valve is connected at contacts C, 2, and O. Thepair of contacts at C and 2', or at 2 and O, is used depending uponwhether the solenoid valve is to be normally open or normally closed. Asshown, the power supply is grounded through line 112, to which line 59is also connected.

6 The invention may 'be illustrated by the following examples.

Example 1 A cooling device was constructed subtantially in the mannerillustrated in FIG. 1, with l-gallon plastic foaminsulated vesselsserving as bath container and Dry Ice reservoir. A 7-inch long heatexchange tube (corresponding to 26 in FIG. 1) was fabricated from A-inch O.D. copper tubing, as were the conduits corresponding to 23 and 27in FIG. 1. A solenoid valve was placed in the upper return conduit andflow regulation was accomplished by inserting a small ru bbe-r stoppercontaining a small orifice in the discharge end of the upper conduit.This entire assembly, consisting of the two vessels connected byfii-inch O.D. copper tubing and the solenoid valve, was placed in aninsulated tank with inside dimensions of 17 /2" L X 8 /2" W x 11" D andoutside dimensions of 19" L x 10" W x 12" D. A small screen was placedover the lower inlet tube in the Dry Ice reservoir. A thermoregulatorwas placed in the bath container and, through an electronic relay,operated the solenoid valve. The regulator, relay, and valve wereexactly as shown in FIG. 2. The lbath container was filled withmethanol, and Dry Ice was placed in the other vessel. Methanol waspoured over the Dry Ice. The thermoregulator was set for a temperatureconsiderably below that of the methanol in the bath container so thatthe solenoid valve remained open. Within two minutes after addition ofmethanol to the Dry Ice, cold methanol-Dry Ice solution begancirculating through the conduit and the :bath liquid began to cool. Sometypical cooling rate data obtained for this system are:

Time, p.m.: Temp, deg. F.

Time, am.

Additional cooling rate data were obtained by measuring the temperaturedrop over a short time interval with a stop watch. The rate at 38degrees F. was approximately 0.5 degrees F./min., and at 65 degrees F.it was 0.15 degrees F./min. By placing in the discharge linecorresponding to line 27 of FIG. 1 a small stopper having an orifice ofabout inch diameter, bath temperature could be controlled toplus-or-minus 0.02 degree F. at 0 degree F., -38 degrees F., and -65degrees F. The stopper functions as the hand throttle valve 31 of FIG. 1but does not, of course, have the flexibility of the latter.

Example 2 A bath identical to the construction shown in FIGS. 1 and 2(except for the window in the bath compartment and the remote locationof relay 43) was constructed. This unit had outside dimensions of 16" Lx 8" W x 10 /2 D, and the bath container and Dry Ice reservoir eachmeasured 5%" L x 4 /4 W x 8 /2" D. The heat exchange tube 26 wasapproximately 7" long and was fabricated from OD. stainless steeltubing. The unit operated at 0 degree F. and at -65 degrees F. with acontrol better than plus-or-minus 0.02 degree F. The rate of cooling ofthe unit was of the order given in Example 1.

It may be seen that the invention makes available a bath of improvedcompactness and accessibility of parts. Control over the rate of coolingis provided without necessity for the presence of a heater, thussimplifying the control operation and conserving coolant. Undercoolingis avoided, resulting in shortening of the time necessary to attaintemperature equilibrium. The bath further incorporates the feature ofself-regulation of the cooling step, whereby cooling is self ceasingthrough the action of carbon dioxide gas in quickly building up backpressure in tube section 26.

It is noteworthy that, with bath compactness achieved, resulting indecreased heat capacity in the bath, very good temperature control wasattained.

Other noteworthy features include the wide variation that is possible inthe coolant flow rate, i.e., a large flow rate is available for rapidprecooling of the bath and a very small flow rate for precisetemperature control. Furthermore, all of the action is carried outwithout a mechanical or other pump.

Liquid carbon dioxide may be used instead of Dry Ice. Additionally,liquid helium, liquid oxygen, liquid nitrogen, liquid argon, and in factany other normally gaseous material which can be liquefied at a suitablelow temperature, including the Freons" (a group of halogenatedhydrocarbons containing one or more fluorine atoms), may be used. Liquidcarbon dioxide under pressure is suitable. These fluid colants may beused With or without a solvent.

Other temperature sensing devices may be substituted for thethermoregulator, such as a resistance thermometer, thermistor, igasthermometer, bi-metallic strip, etc., with appropriate circuitry, forcontrolling the constant temperature bath. 7

The invention is applicable to other systems besides a constanttemperature bath. For example, if bath compartment 11 contains, say, airinstead of liquid, then the device may constitute an environmentalchamber suitable for maintaining sensitive substances like tissue speci-3 mens, bacterial cultures, etc. at a closely controlled lowtemperature. Frozen foods may similarly be maintained at a constant lowtemperature. Additionally, the size of the device may be increased asdesired, even to that of a walk-in chamber. In these applications, thelid of course would be replaced by a non-apertured air-tight panel orwall. Or the container 11, having any suitable type of fluid therein,may be used for other purposes, including application in a freeze dryingsystem and the like.

The device of the invention is sometimes termed a bath, or a constanttemperature bath, or a constant temperature environmental device.

It will be understood that the invention is capable of obviousvariations without departing from its scope.

In the light of the foregoing description, the following is claimed.

1. A constant temperature device comprising a bath section for holdingbath liquid, a reservoir section for holding coolant solution, and acentral section intermediate said first two sections, said coolantsolution being maintained out of direct contact with the bath liquid,switching means associated with said central section, said bath and saidreservoir sections having the same floor level and substantialy the sameliquid levels, temperature sensitive means in said bath liquid, meansfor transferring coolant laterally from the reservoir section to thebath section for indirect heat exchange therewith and means fortransferring the coolant laterally back to the reservoir section, saidtransfer means having a valve disposed therein responsive to saidswitching means for controlling coolant flow, said switching means beingactuatable by said temperature-sensitive means to operate said valve,said valve being operative to open to permit coolant flow therethroughwhen the temperature-sensitive means signals a high bath liquidtemperature and to close to shut ofl coolant flow when thetemperature-sensitive means signals 310W bath liquid temperature.

2. A constant temperature device comprising a bath section for holdingbath fluid, a reservoir for holding fluid coolant, and a central sectionintermediate said first 0 two sections, electrical switching meanssupported by said central section, said bath and reservoirsectionshaving the same floor level and substantially the same fluidlevels, temperature-sensitive means in said bath section, means fortransferring coolant laterally from the reservoir section to the bathsection and back again, said transfer means comprising a lower inletconduit and an upper outlet conduit both connected by an upwardlydirected conduit, said upper conduit having electrically operable valvemeans disposed therein for controlling coolant flow and being located insaid central section, said switching means being actuatable by saidtemperature-sensitive means to operate said valve means, and saidupwardly directed conduit being disposed in said bath fluid.

3. A constant low temperature device comprising a container having abath fluid which is to be maintained at a constant temperature belowroom temperature, a conduit of heat-conducting material extending intothe container in heat exchange relation with said fluid, a reservoir ofa low boiling carbon dioxide solvent adjacent the conduit and laterallyspaced from the container, ends of the conduit extending into saidreservoir, 21 mass of solid carbon dioxide resting in the solvent anddissolving therein to form a cold solution, said solution being adaptedto flow from the reservoir into the conduit and thereby cool the bathfluid, the heat of the bath fluid acting to boil out dissolved carbondioxide from the solution, said conduit conducting the resulting carbondioxide gas to said reservoir, said conduit in turn being refillable bycold solution from the reservoir, means in the conduit to reduce theflow in the conduit, said means being responsive to the temperature ofsaid bath fluid, and a second means in the conduit manually operative toreduce the flow in the conduit,

d. The device of claim 3 wherein one of said conduit ends is disposedbeneath the level of solution in said reservoir and the other of saidends is above said level.

5'. A constant temperature device comprising a bath section for holdingbath fluid, a reservoir section for holding coolant,temperature-sensitive means in the bath section, coolant conduit meansextending from the reservoir section into and through the bath sectionand back to the reservoir section and comprising (1) an inlet portion toreceive coolant from the reservoir, (2) a heat transfer portion disposedgenerally vertically in the bath section, and (3) an outlet portion forreturning coolant to the reservoir section, and flow control means insaid outlet portion responsive to said temperature-sensitive means forcontrolling the flow of coolant therein, said coolant comprising Dry Iceand a solvent therefor, the solvent dissolving the Dry Ice to form acold saturated solution which functions as said coolant, said solutionflowing from said reservoir section into said conduit means and actingto cool said bath fluid by indirect heat exchange through said heattransfer portion, said solution thereby becoming sufliciently warmedtoboil out carbon dioxide gas, said heat transferportion and outletportion con ducting said gas to the reservoir section and therebyproviding for the flow of additional solution into said inlet portionfrom said reservoir section, and said flow control means, in response tosaid temperature-sensitive means when the bath fluid is cooled, actingto close off the flow of gas in said outlet portion.

References Cited by the Examiner UNITED STATES PATENTS 1,989,247 1/1935Rooney 62-384 X 2,040,744 5/1936 Hull 62-167 2,170,332 8/1939 Justheim62l68 2,359,796 10/1944 Russell 62l68 X 2,835,477 5/1958 Tourog 62-167 X3,163,022 12/1964 Hottenroth 62-168 MEYER PERLIN, Primary Examiner.

1. A CONSTANT TEMPERATURE DEVICE COMPRISING A BATH SECTION FOR HOLDINGBATH LIQUID, A RESERVOIR SECTION FOR HOLDING COOLANT SOLUTION, AND ACENTRAL SECTION INTERMEDIATE SAID FIRST TWO SECTIONS, SAID COOLANTSOLUTION BEING MAINTAINED OUT OF DIRECT CONTACT WITH THE BATH LIQUID,SWITCHING MEANS ASSOCIATED WITH SAID CENTRAL SECTION, SAID BATH AND SAIDRESERVOIR SECTIONS HAVING THE SAME FLOOR LEVEL AND SUBSTANTIALLY THESAME LIQUID LEVELS, TEMPERATURESENSITIVE MEANS IN SAID BATH LIQUID,MEANS FOR TRANSFERRING COOLANT LATERALLY FROM THE RESERVOIR SECTION TOTHE BATH SECTION FOR INDIRECT HEAT EXCHANGE THEREWITH AND MEANS FORTRANSFERRING THE COOLANT LATERALLY BACK TO THE RESERVOIR SECTION, SAIDTRANSFER MEANS HAVING A VALVE DISPOSED THEREIN RESPONSIVE TO SAIDSWITCHING MEANS FOR CONTROLLING COOLANT FLOW, SAID SWITCHING MEANS BEINGACTUATABLE BY SAID TEMPERATURE-SENSITIVE MEANS TO OPERATE SAID VALVE,SAID VALVE BEING OPERATIVE TO OPEN TO PERMIT COOLANT FLOW THERETHROUGHWHEN THE TEMPERATURE-SENSITIVE MEANS SIGNALS A HIGH BATH LIQUIDTEMPERATURE AND TO CLOSE TO SHUT OFF COOLANT FLOW WHEN THETEMPERATURE-SENSITIVE MEANS SIGNALS A LOW BATH LIQUID TEMPERATURE.